Energy Changes and Electron Transfer in Metabolism: Redox Reactions
Chapter 15
Outline
• standard states for chemical reactions • a brief discussion of metabolism
• the role of redox reactions in metabolism • the role of coenzymes in redox reactions • coupling reactions to achieve a –ΔG • coenzyme A
• I really do like our textbook…
- I think it’s easy to read (as easy as a biochem text could be) - I think most of the figures are helpful
- but, I also think it’s organized well
• REDOX reactions are centrally important to ____________________________
• redox reactions are the chemistry that produce the energy for making ATP – no ATP, no ______ • students often struggle with redox reactions because they’re introduced within metabolism –
when a million other things are going on
• our text has chosen to introduce redox reactions now to get you familiar with them all by themselves
• we’ll follow that lead
Standard States for Free Energy Changes
• before we discuss redox reactions, we need to get a bit more background on general chemistry
- that included STANDARD ________________
• we can define standard conditions for any process, experiment, reaction, etc.
- this includes temperature, atmospheric pressure, ______________________ of reactants, ___ and other parameters
• what those things are (30°C, 1 atmosphere, 1M) is arbitrary - we pick it; people decide what to use
• what’s important is that they are STANDARDS – everyone uses the same standards for their experiment
• this way, a researchers in Denver, CO can compare his/her results to mine – we each did the same experiment the same way
• when a reaction is being done under standard conditions, each individual component of the reaction is said to be in its STANDARD STATE
• for _______ solids and liquids (elemental), the purity of the substance itself is its standard state
• for an elemental gas, the standard state is that gas at 1 __________________________ • for solutions, the standard state is a 1M concentration of that solution
• whenever you see a super script zero (°) on chemical terms, that is telling you that the value is valid under standard conditions
Modifying the Standard State for Biochemistry
• the defined standard state for chemistry dictates that all solutions be at a ______ concentration • anybody see a problem with this in biochemistry…?
• very rarely is anything in a living system at such a high concentration
- for example, if [H+] equaled _____, the pH of that solution would be zero! • the standard state used for chemistry is not suitable to living things
- so we define our own standard state
• if we deviated from chemistry’s standard state too much, then chemistry and biochemistry would be doing things so differently we couldn’t learn from each other’s successes
• so, the only significant difference between the standard state of chemistry and biochemistry is pH
- the standard state of biochemical reactions includes a pH of ____
• this modified standard state is given the designation superscript zero prime - ΔG°’ (said “delta g
zero ________________”)
Metabolism: A Brief Overview
• its been said that sharks eat, swim, and make little baby sharks • its been said that infants eat, poop, cry and sleep
• these statements are both true… • in fact, all that livings things really do is:
- eat - poop - procreate
• ALL our behaviors – all the behaviors of all living things – can be linked to one of those three motivations
• I’ll leave the last one to your imagination • let’s consider eating and pooping
- from the biochemical perspective, of course… • eating and pooping
• taking in stuff we need and getting rid of the stuff we don ‘t
• building the molecules required for life and disposing of the unwanted, unusable, or harmful byproducts
- all the same thing said different ways – don’t ever forget that
• these processes – breaking down what we eat, building what we need, and disposing of the
waste – are collectively called ____________________________
• breaking down large ‘macromolecules’ that we eat (proteins, complex sugars, fats) into their
individual subunits is called _______________________
• taking small molecules – individual building blocks; subunits – and using them to build larger,
complex molecules needed by the organism is called ___________________________
• catabolism and anabolism are NOT the ________________ of each other
- that’s at the heart of consumption – gaining energy from the foods we eat
- this is an _______________________ process (more on that in a little bit)
• anabolism – building things we need – _______________ energy - building anything takes energy because it flies in the face of entropy
- this is a ________________________ process (more on that soon, too)
• oxidation and reduction reactions (REDOX reactions) are simply reactions that involve the
transfer of __________________
- but with these ____________________, energy is also transferred and this is why we see redox reactions in metabolism
- moving energy around from one molecule to another
Redox Reactions and Their Role in Metabolism
• we’re talking about moving electrons from donors to acceptors
• when a molecule loses or donates electrons, its been ___________________
• when a molecule gains or accepts electrons, its been ___________________ (I always remember this because it’s counter-intuitive…)
• a compound that causes another molecule to become reduced is called a REDUCING AGENT • β-mercaptoethanol reduces disulfide bridges between cysteine residues in a protein
- BME is a REDUCING AGENT
• there are also OXIDIZING AGENTS that (obviously) cause other molecules to be oxidized
• these ‘agents’ are critical for metabolism and forcing _________________ to move • let’s put some zinc (Zn) in a solution of copper ions (Cu2+)
- why…? to say we did…
• after a little while, the zinc disappears and copper metal is deposited (falls out of solution) - what the hell happened…?!
- where ‘s’ is solid and ‘aq’ is aqueous (in a solution of water)
• the zinc metal ____________ two electrons in this reaction
- by _____________ two (negative) electrons, zinc is left with a +2 charge
- it was ____________________
• copper ________________ two electrons, neutralizing its charge
- it was _________________ • this is an entire redox reaction
- ___________ is reduced
• we often depict redox reactions in HALF-REACTIONS - the half reaction of reduction
- the half reaction of oxidation
• we can also consider zinc to be the reducing agent and copper to be the oxidizing agent • this is the simplest form of a redox reaction
- far simpler than anything that occurs in a living cell
• but, it is useful to understand the roles played by electron donors and acceptors – as well as the movement of electrons themselves
The Role of Coenzymes in Redox Reactions
• you’ve probably all covered redox reactions before in chem classes - but the redox reactions that occur in living systems are different
• living things use redox reactions as a mechanism for shuttling _____________________ • also, we’re not chemists – so we will not be discussing redox reactions in terms of numbers
- we’ll discuss it for what it is – the movement of electrons - we’ll also keep it simpler by using half-reactions
• this introduces another useful ‘hint’
• in biological redox reactions, where electrons go, _______ goes • what we don’t see in this half-reaction is the oxidizing agent
- what got reduced in this redox reaction?
• very often, the role of a coenzyme (_____________________) is to be an ‘agent’ in a redox reaction (either one…)
• nicotinamide adenine __________________ (NAD - derived from niacin) is one such coenzyme • NAD+ can be reduced to NADH
• NADH can be oxidized to NAD+
• the gain of a proton in the reduced form pushes bonds and __________________ the positive charge on the nitrogen
• if NADH were the reducing agent and it was mixed with acetaldehyde, we might see the following reaction
• it is important to note that if ethanol were mixed with NADH, nothing would _____________ – EVER
• ethanol is fully _________________________
- adding a reducing agent isn’t going to accomplish anything • acetaldehyde is capable of being reduced
- add a reducing agent like NADH and chemistry will happen • this redox reaction releases free energy
- what kind of ΔG does it have…? - is it spontaneous….?
- will it happen instantaneously…?
• ____________
- and that’s why we need an enzyme!
• NAD+/NADH is not the only coenzyme capable of redox • NADPH is related to NADH, but with an extra phosphate group
- NADPH is used for specialized redox reactions
- FAD can carry two protons and two ____________________ - FAD is reduced to FADH2
• oxidizing the food we eat – to milk out of it every last drop of energy possible – requires that many things become reduced
- requires that many things accept electrons from that food
• the “ultimate electron acceptor” is __________________
- _________________ accepts all those electrons – and the protons that come with them – to
become ______________
- this is why we _______________
- to get all the _________________ we need for metabolism
• but along the way to oxygen, many intermediates are reduced and oxidized (can’t ever have the re- without the –dox)
- those electrons change hands many many times
• this process of getting energy out of food and into ATP by redox reactions will consume seven of the next nine lectures
- but first, some more general concepts of metabolism
Coupling Reactions to Achieve Thermodynamic Favorability • we’ve established that oxidizing molecules releases energy
- but that released energy is worthless unless we can ____________ it eventually, this energy will be locked and stored in ATP
- ATP contains a __________________ bond (the bond holding the last phosphate group to the molecule)
- by making ADP (A-Di-P) into ATP (A-Tri-P), we store energy in that ___________________ bond • then, when energy is needed for biological processes, that energy can be released by
hydrolyzing ATP to ADP • and so, it is an ongoing cycle
- eat and release energy by ______________________ - store and ship that energy as ATP
- release that energy to build things
• if two reactions are linked by shared products and reactants… - those reactions are said to be COUPLED
• coupled reactions have a very interesting and useful property
- the ΔG of the coupled reactions is the _________ of the ΔGs for the individual reactions • let’s say that a little bit more clearly
- if an unfavorable reaction (+ΔG) is coupled to a favorable reaction (-ΔG) – and the total ΔG is negative – the entire process will be thermodynamically favored
- the universe will tolerate the +ΔG as long as a sufficiently high –ΔG is also there to offset it
Coenzyme A: Activating Nonreactive Molecules
• oxidizing food molecules into energy and water takes many steps - sometimes the molecules involved in these steps “don’t want to react”
• in these cases, the non-reactive molecule is linked (bound) to a reactive molecule (often a coenzyme)
- when the bond between these two things is then _________________, energy is released
which can drive _________________ reactions forward through ________________
A + Coenzyme → A-Coenzyme ___________________ Step A-Coenzyme + B → AB + Coenzyme Favorable Reaction
• making a molecule more reactive in this way is called _______________________
• coenzyme A (CoA) is often used in ______________________ to make things happen that wouldn’t otherwise occur
• CoA contains a very reactive ____________ group (same group as on cysteine) • it is through this group that CoA bonds with other, nonreactive things
• when this bond is broken, energy is released making a nonreactive molecule involved in a reactive process
• one of the most important molecules we’ll meet later this semester is acetyl-CoA • but, that’s for later…
Summary
• ΔG° is telling you the value for the _____________ energy __________________ of a reaction
when that reaction is performed under ________________ conditions
• ΔG°’ is the free energy change of a reaction under standard conditions where the pH is ____ • breaking down what we eat, building what we need and disposing of the waste is collectively
called _____________________
- breaking down large macromolecules - ______________________
- taking small molecules and using them to build larger, complex molecules – _______________ • oxidation and reduction reactions (REDOX reactions) are reactions that involve the transfer of
__________________ - _________________ is also transferred
- when a molecule loses or donates electrons, its been ________________
- when a molecule gains or accepts electrons, its been ___________________ • we discussed NAD+/NADH and FAD/FADH2 – coenzymes that are redox facilitators
• ____________________ can achieve an overall __________ and drive reactions forward
• _____________________ makes nonreactive molecules reactive (CoA is often used)
